1. Field of the Invention
The present invention relates to acoustic wave filter devices utilizing surface acoustic waves or boundary acoustic waves. In particular, the present invention relates to acoustic wave filter devices having first and second longitudinally coupled resonator type acoustic wave filters disposed on piezoelectric substrates and also having a balanced-to-unbalanced conversion function.
2. Description of the Related Art
Conventionally, surface acoustic wave filters have been in widespread use as band pass filters for mobile communication apparatuses such as mobile phones and the like. Recently, boundary acoustic wave filters utilizing boundary acoustic waves instead of surface acoustic waves have drawn attention.
Band pass filters used in the RF stages of mobile phones are required to have balanced-to-unbalanced conversion functions. By using boundary acoustic wave filters having balanced-to-unbalanced functions as band pass filters, baluns can be omitted, which promotes miniaturization.
Japanese Unexamined Patent Application Publication No. 2002-374147 discloses a surface acoustic wave filter having a balanced-to-unbalanced conversion function.
FIG. 8 is a plan view illustrating an electrode structure of a surface acoustic wave filter described in Japanese Unexamined Patent Application Publication No. 2002-374147. A surface acoustic wave filter 501 has a configuration in which the shown electrode structure is formed on a piezoelectric substrate. In FIG. 8, first to third IDTs 502 to 504 are arranged along a surface acoustic wave propagation direction. Reflectors 505 and 506 are arranged along a surface acoustic wave propagation direction, on opposite sides of the area in which the IDTs 502 to 504 are provided.
A first end of the IDT 503 provided at the center is connected to an unbalanced terminal 507 and first ends of the IDTs 502 and 504 provided at opposite sides of the IDT 503 are electrically connected to first and second balanced terminals 508 and 509, respectively.
In the surface acoustic wave filter 501, the IDTs 502 and 504 are configured such that a signal supplied from the unbalanced terminal 507 to the balanced terminal 508 and a signal supplied from the unbalanced terminal 507 to the balanced terminal 509 have opposite phases. The IDT 502 is cross-width weighted so that the balance is improved.
Conventionally, in a surface acoustic filter having a balanced-to-unbalanced conversion function, there is a strong requirement to improve the balance between balanced signals input/output from first and second balanced terminals. In the surface acoustic wave filter 501 described in Japanese Unexamined Patent Application Publication No. 2002-374147, as described above, cross-width weighting is applied to the IDT 502 connected to the balanced terminal 508 with a view to improving the balance. Specifically, weighting is applied to at least one IDT 502, so that reception states of surface acoustic wave excitation in signals output from the IDTs 502 and IDT 504 are matched.
However, it is difficult to match the amount of surface acoustic wave excitation received at the IDT 502 side and the amount of surface acoustic wave excitation received at the IDT 504 side, making it difficult to sufficiently and precisely enhance the balance.
In addition, it is also difficult to design weighting that allows the amounts of surface acoustic wave excitation received at the IDTs 502 and 504 to be matched.